Number 141, August 25, 1993 by Phillip F. Schewe and Ben Stein
WHY DON'T MORE SUPERNOVAS LEAVE BEHIND PULSARS? The venerable Hans Bethe,
pioneer in the study of stellar nuclear physics for decades, and Gerald
Brown of Stony Brook, propose that some collapsing stars---with masses
as low as 18 solar masses---refrain temporarily from vanishing into a black
hole and instead pause at the neutron-star level of compactification long
enough to trigger a supernova explosion. The residual core would thereafter
shrink without a trace into a black hole. Bethe and Brown invoke a hypothesis
about neutron stars introduced in the last few years by Brown and others.
According to this model, as the density at the core of a collapsing star
exceeds three times the density of ordinary matter, electrons might not
necessarily combine with protons to form neutrons, as was thought, but
might instead spawn K mesons and neutrinos. In place of the traditional
pure neutron matter, the collapsing core would then be a more compressible
mixture of protons and neutrons, buoyed up for a time by the restless motions
of the neutrinos. When they depart the collapse would continue, leaving
behind either a "nucleon star" or a black hole. (Science, 13
August 1993.)
WHAT MAKES AN HOURGLASS TICK? Scientists have discovered that the sand
grains in an hourglass flow continuously only in those hourglasses with
the proper ratio of neck width to grain diameter. For ratios outside the
proper range, the hourglass "ticks": the grains alternate between
flowing and not flowing at a constant rate. Researchers at the University
of Pittsburgh (contact Xiao-lun Wu, 412-624-0873) and the University of
Rennes in France have found that this "ticking" effect is caused
by tiny pressure differences--typically, 1/10,000 of an atmosphere--between
the upper and lower halves of the hourglass, and also by the formation
of arch-like structures in the grains. Small external disturbances can
alter the ticking rate--or stop the flow entirely. This knowledge can potentially
be exploited to manipulate granular materials in silos and hoppers, which
have geometries similar to that of an hourglass. In addition to showing
the importance of air in an hourglass, this experiment demonstrates the
tendency of granular materials to act as a collective substance--in this
case, through the formation of arches--rather than as individual particles.
This collective state of matter is neither a solid nor a liquid. For example,
a normal liquid would flow more slowly through an hourglass as level decreased.
(X-l. Wu et al, Phys. Rev. Lett, 30 August 1993.)
THE WORLD IS MADE MOSTLY OF GLUE , according to Nathan Isgur, insofar
as the quarks which make up the nucleons which make up atomic nuclei can
be thought of as light "bare" quarks endowed with greater mass
through the agency of gluons, the carriers of the strong nuclear force.
Isgur is the chief theorist at the Continuous Electron Beam Accelerator
Facility in Virginia, which will be completed in 1994. At CEBAF 6-GeV (rather
than the originally planned 4-GeV) electrons will scatter from nuclei in
order to explore an interaction regime not often examined in higher-energy
experiments, a regime in which the interaction of quarks through the exchange
of gluons and the interaction of nucleons through the exchange of mesons
are both important. (Physics Today, August 1993.)
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